Abstract
The synaptic connections between neurokinin 1 (NK1) receptor-like immunoreactive (LI) neurons and γ-aminobutyric acid (GABA)-, glycine (Gly)-, serotonin (5-HT)- or dopamine-β-hydroxylase (DBH, a specific marker for norepinephrinergic neuronal structures)-LI axon terminals in the rat medullary dorsal horn (MDH) were examined under electron microscope by using a pre-embedding immunohistochemical double-staining technique. NK1 receptor-LI neurons were observed principally in laminae I and III, only a few of them were found in lamina II of the MDH. GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were densely encountered in laminae I and II, and sparsely in lamina III of the MDH. Some of these GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were observed to make principally symmetric synapses with NK1 receptor-LI neuronal cell bodies and dendritic processes in laminae I, II and III of the MDH. The present results suggest that neurons expressing NK1 receptor within the MDH might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons located in the MDH and 5-HT- or norepinephrine (NE)-containing descending fibers originated from structures in the brainstem.
Highlights
The medullary dorsal horn (MDH) receives inputs from smalldiameter thin-myelinated Ad and unmyelinated C fibers that convey preferentially nociceptive information from the orofacial structures mainly through cranial nerves, such as trigeminal (V), facial (VII), glossopharyngeal (IX) and vagus (X) nerves [1]
Contacts revealed by immunofluroscent labeling Many neurokinin 1 (NK1) receptor-like immunoreactive (LI) neurons with well-developed dendritic processes were observed in the medullary dorsal horn (MDH)
Some large NK1 receptor-LI neurons were found in lamina III with their long dendritic processes extending dorsally into lamina II and some of them even occasionally extending into lamina I
Summary
The medullary dorsal horn (MDH) receives inputs from smalldiameter thin-myelinated Ad and unmyelinated C fibers that convey preferentially nociceptive information from the orofacial structures mainly through cranial nerves, such as trigeminal (V), facial (VII), glossopharyngeal (IX) and vagus (X) nerves [1]. The superficial laminae of the dorsal horn receive dense descending inputs, including serotoninergic and norepinephrinergic systems, originating from the rostral ventromedial medulla (RVM), including the nucleus raphe magnus (NRM) and its surrounding reticular formation and locus ceruleus in the brainstem, respectively [11,12] These descending systems act on both presynaptic and postsynaptic sites to control the gain of neuronal excitability in nociceptive transmission [13,14]. In the spinal and medullary dorsal horns, neurokinin 1 (NK1) receptor (SP receptor)-like immunoreactivities have been found principally in laminae I and III [16], from which the major pathways relaying noxious information to the thalamus, namely the spinothalamic tract and trigeminothalamic tract, originate [1] These results suggest that NK1 receptor-containing neurons in the superficial laminae might receive nociceptive information conveyed by SPcontaining primary afferent fibers and transmit it to the thalamus, fall into the projection neurons category
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